Do Striga hermonthica-induced changes in soil matric potential cause the reduction in stomatal conductance and growth of infected maize plants?

Maize ( Zea mays L.) plants parasitized by the root hemi-parasitic angiosperm, Striga hermonthica (Del.) Benth., consistently display a range of symptoms similar to those found in droughted plants. The mechanisms by which these changes occur are largely unknown. However, S. hermonthica has unusually high rates of transpiration, and stomata which are relatively insensitive to water deficit. Consequently, it has often been suggested that the parasite might cause a severe depletion of the available water in the host's rooting zone. To determine whether the lower stomatal conductance and retarded growth of infected plants could be a result of parasite-induced water deficit, we have monitored the matric potential of the growth medium, water use, growth and stomatal conductance of infected vs. uninfected maize plants.
Host plant height and stomatal conductance of parasitized plants were significantly lower than those of control plants from 31 or 37 d after planting (d.a.p.) respectively. However, there was no indication of an increase in the rate of water depletion in the rooting zone of infected plants until approx. 63 d into the parasitic association. In fact, from 39 until 59 d.a.p. infected plants used less water than uninfected control plants, probably the result of the plants having fewer expanded leaves during part of this period, combined with the lower stomatal conductance exhibited by the infected plants from day 37 onwards. Leaf RWC of infected plants was unchanged in comparison with that of uninfected plants, therefore the change in stomatal conductance was not a response to dehydration of the leaf tissue. Our results indicate that parasitism by S. hermonthica does not cause an increase in water uptake/use in the host until well after most of the symptoms of infection have become fully established. It is highly unlikely, therefore, that the observed effects on the host are primarily due to soil water deficit.     

The Combined Effects of Desiccation and Irradiance on Mosses from Xeric and Hydric Habitats

The combined effects of desiccation and irradiance on the physiologyof the sand dune moss Tortula ruraliformis (Besch.) Grout andthe minerotrophic flush moss Dicranella palustris (Dicks.) Crundw.ex. E. F. Warb (D. squarrosa (Starke) Schp.) were studied. Damageas a result of desiccation in the dark, measured by loss ofprotein and the relative accumulation of thiobarbituric acid(TBA) reactive products (which gives an estimation of lipidperoxidation), was greater in D. palustris. Desiccation alonehad no effect on the total concentrations of chlorophyll andcarotenoids in either species. Water loss resulted in the cessationof measurable photosynthetic oxygen evolution in both species.Respiration was less sensitive to desiccation than was photosynthesis.A combination of irradiance and water stress prevented any recoveryof photosynthesis during subsequent rehydration in D. palustris,but suppressed recovery only marginally (at the highest irradiance)in T. ruraliformis. The loss of protein, chlorophyll, and carotenoids,and lipid peroxidation were all substantially increased in D.palustris desiccated in the light, but these same conditionsresulted in only minimal damage of T. ruraliformis. Continuousexposure to high irradiance was less deleterious to desiccatedthan hydrated T. ruraliformis. The data are discussed in relationto the habitat preferences of the two species, and also in relationto possible causal factors in the initiation of damage. Key words: Desiccation, mosses, oxidative damage, photo-oxidation     

Effects of Desiccation on some Activated Oxygen Processing Enzymes and Anti-Oxidants in Mosses

The possibility that desiccation tolerance in mosses may bedependent on an ability to process species of activated oxygenwas investigated using the tolerant sand-dune moss Tortula ruraliformis(Besch.) Grout and the sensitive minerotrophic flush speciesDicranella palustris (Dicks.) ex. E. F. Warb (D. squarrosa (Starke)Schp.). Mosses were desiccated in low and high irradiance andresponses of these plants compared to those of dark-desiccatedand hydrated control plants. Both desiccated and undesiccatedplants of T. ruraliformis had a higher superoxide dismutase(SOD) activity than D. palustris, but had similar, or loweractivities of the chloroplastic H₂O₂-processing enzymes peroxidaseand ascorbate peroxidase. In T. ruraliformis, desiccation inboth light and dark led to a significant increase in SOD activity,but did not consistently stimulate the activities of peroxidaseand ascorbate peroxidase. In D. palustris, desiccation in combinationwith irradiance led to a decrease in peroxidase activity, buthad little affect on the activities of other activated oxygen-processingenzymes. Catalase, an extra-chloroplastic enzyme, was up to7-fold more active in hydrated T. ruraliformis than in D. palustris,but desiccation resulted in significant decreases in the activityof this enzyme in both species. Regardless of irradiance level,there was a depletion of the anti-oxidant ascorbic acid in bothspecies when desiccated. Only in T. ruraliformis was there asynthesis of     

NUMBERS OF MEADOW PIPITS ANTHUS PRATENSIS ON MOUNTAIN FARM GRASSLAND IN NORTH WALES IN THE BREEDING SEASON

On c. 15 ha of mountain farm grassland in Wales, during 1972-75, adult Meadow Pipits were present during March–September (but scarce in August–September). Breeding occurred during late April to late July. Incubation was performed by the female but both sexes fed the nestlings. Adults showed an average annual mortality of 54%. Adults spent nearly all their time on the ground, but varied in their activities and use of the habitat according to the stage of breeding. Their unobtrusive behaviour had a marked effect on the ease with which they could be observed for a population count. Males were seen on 30% of the days for which they were assumed to be present and were recorded giving song-displays on 28% of the days when they were actually seen. We conclude that, to count all the birds present on a site, most must be marked, sightings must be carefully mapped and the number of observer days must be at least 20, preferably in April-May. The numbers of individually marked (and a few unmarked) birds were determined by mapping them on large-scale aerial photographs. This gave an average annual density of 48 resident pairs per km². The average home range was 2.18 ha; this value remained relatively constant from year to year. At low population density individual home ranges were separated from one another, and there was some unoccupied ground, but at high population density the ranges overlapped with one another.